Several systems exist to center a radiation beam's incident spots on information track centers, but these other systems have several limitations, and for commercially feasible systems which provide optical recording densities on the order of 1000M bytes to a 30 cm disk, a highly accurate system must be deviced.
In the preferred form of this invention, a single beam is directed at an angle to the disk or recording medium surface, and a detector receives the reflected beam which has been modulated by the disk surface. Parsing the signal generated by the detector means yields accurate information concerning the location of the beam relative to the track center and concerning the data in the track. In another "seek" mode, the number of tracks crossed can be parsed from the same signal.
The concepts disclosed herein may be applied to the use of a separate beam for writing, multiple detector beams, or splitting the reflected beam between a multiplicity of detectors, provided that the format limitations on the configuration of track sector headers on the recording medium surface are adhered to and/or that such format limitations are used in accord with the methods described for track following or track counting.
The preferred embodiments employ a reflective media surface, physically responsive to radiation (of laser light for these embodiments). However, the invention can apply to various forms of media, including reflective and transmissive, and physically or chemically radiation responsive media so long as the novel and useful structure described herein is employed.
Generally, as is the case in the preferred embodiments described herein, disk media is "mastered," created with data tracks (grooves), non-data areas (generally lands surrounding the grooves) and servo or adjustment areas (called headers, situate at spaced locations in and about the centerline of the grooves). These are all in the information layer or plane of the disk. Disk media may be "mastered" with data too, and completely blank disk media may at some future date be sold for use in optical drives which do the mastering themselves, using the same equipment which reads and writes data.
The inventive concepts described herein may apply to each of these media forms, but in the Detailed Description of Preferred Embodiments only one form is described and the information bearing layer's detectable modulations due to writing are therein referred to as "pits" although these pits may be bumps or other manifestations when a different media is employed.
One existing type of system for centering a beam of radiation in a track is shown in U.S. Pat. No. 4,271,334 which provides for the dithering or wobbling of the beam (or related beam) within or across the width of the track as the track passes. An error signal is produced using the increase in the reflected beam's average intensity (which increases as the beam gets farther off-center), and the fact that the reflected beam's intensity variation produces a phase angle with the dither signal on only one side of the track. The amount of increased intensity reflects the magnitude of the off-center error and the direction of the off-center error is found in the existence or nonexistence of the phase angle between the dither signal and the reflected intensity variation signal. U.S. Pat. Nos. 4,236,105 and 4,234,837 describe a dither system which uses "switching lines" to signal the servo mechanism to change direction. Dithering, or active wobbling, has inherent design problems however, which prevent its easy implementation in write/read systems.
In U.S. Pat. No. 4,243,850, the tracking error signal is generated by the use of three read beams' spots in which the outer two spots' reflections gain or lose intensity when they come in contact with the information pits or hills of adjacent tracks. This signal is a differential signal generated by paired photodetectors which read these outer reflected beams, the absolute value of the difference showing the magnitude of the error and the fact of a positive or negative difference indicating the direction of the error.
Other systems employ the diffraction of light by track edges themselves to generate a track following error signal called a push-pull signal described in U.S. Pat. Nos. 4,232,337; 4,209,804 and 4,100,557. Difficulties with these systems are discussed in more detail below but basically inaccuracies in beam alignment cause undiscoverable flaws in the push-pull signal, making it an inaccurate measure of tracking.
Other systems use an error signal generated by the disk track's surface structure wobbling with respect to the line of information pits embedded in the center of the track. This error signal may be generated by the sinusoidal variations caused by a wobbling groove in which the data pits lie on a straight path as described by U.S. Pat. No. 4,135,083 (at the top of column 8), or by a series of off-center prewritten data pits spaced continuously around the track on either side of the data path center line at predetermined intervals as in Netherland's Pat. Nos. 8,000,121; 8,000,122; 8,103,117 and 8,102,621. In using continuous "passive" wobbling techniques such as these, while they do eliminate the problems associated with active wobble or dithering techniques, the retrieval or parsing-out of the tracking signal (given at the wobble frequency) may be difficult, primarily because the relevant beam spot must first be in track to get a phase lock onto the wobble frequency, and also because of poor signal to noise ratios. In those where many pits are required for timing or track following, rather than wobbling the groove itself, as is required by U.S. Pat. No. 4,456,981, the amount of disk space available for data may be reduced because data cannot be written in the groove adjacent to such wobbled pits. The only abbreviated wobble pit pattern found in the extant art was in U.S. Pat. No. 4,428,069 which did not provide a means for correcting its inaccuracies nor does it, in any way, indicate use of a push-pull signal nor many of the improvements found herein. (The use of wobbled pits in headers for centering has been found in the magnetic recording art too, see for example, U.S. Pat. No. 4,472,750).
One system described a corrected error signal; U.S. Pat. No. 4,476,555. In that patent a "traverse" signal which may roughly correspond to the "central aperture signal" herein is used with a counter and RAM to correct the tracking error signal at a rate of one time per disk rotation, whereas this invention corrects the tracking error signal continuously at each header. Even assuming that the "traverse" signal is a central aperture signal, there is no indication of how it is derived. In the present application, the limitations are taught and claimed which provide for a correcting signal to be generated in the central aperture signal, as well as how to decode the signal to get the corrected tracking information.
Another system for correcting a tracking signal is described in European patent application No. EP0099576A2. That system uses a discontinuity or flat mirror area, and the signal derived therefrom to correct the push-pull signal generated by a track groove. It does not address the problems with signal strength variation caused by written data and reflected light level variations. Neither does it address how to handle errors in location of the blank or "mirror" areas, nor defects around such "mirror" areas.
It should be noted that the diffraction patterns generated by a beam wandering to one side or the other of a track or groove (found in the low frequency push-pull signal) have proven to be unreliable for measuring track following when uncorrected. This is due to shifts in the position of the reflected beam relative to the center of the photodetecting means and the inability to detect what caused the shift. These shifts may be caused by instability in the optics, mechanical displacements, or laser beam intensity distribution itself. This invention solves these problems because the track following signal is a combination of the push-pull signal and a correction signal. In both embodiments the correction signal is derived from the return beam modulated by the header structure of the information track being followed.
One branch of embodiments of the present invention uses a short pattern of wobbled or off-center-line pits or holes combined with the push-pull signal to produce a corrected tracking error signal. The modulation due to the off-center-line pits is found in the central aperture signal, which is derived from the full reflected beam. It also uses the push-pull signal to count track crossings.
The second branch of embodiments of this invention uses the discontinuities in the groove of a track sector header in order to correct the push-pull signal rather than the wobbled pits just mentioned. In this embodiment too, the counting of the changes in the push-pull signal which occur due to the crossing of the beam spot over each track may also be employed to determine relative track address. However, where the invention employs continuous grooves (as in the first mentioned embodiment branch if used without discontinuities) there is no theoretical limitation to track crossing (or seek) speed, whereas there are seek speed limits beyond which an accurate track count may not be possible where tracks are supplied with discontinuities.
A decision relation to which embodiment to use may depend on various considerations including those just described, and extrinsics, such as the cost to produce the system. Of course, the sets of electronics described which decode either the first or the second media embodiment may be included in one system which could work with either of the two basic high data density media structures described.